The present invention relates to a hydraulic brake system with slip control for automotive vehicles comprising a master cylinder actuatable by a hydraulic power booster in which brake system valve means are inserted between the master cylinder and the wheel brakes connected to the master cylinder which allow removal of pressure fluid from the wheel brakes. The pressure fluid taken from the wheel brakes is replenished out of the pressure chamber of the hydraulic power booster. The master cylinder piston is designed as a stepped piston and an annular surface of the master cylinder piston remote from the working chamber is adapted to be acted upon by the pressure of the working chamber. The chamber confined by the annular surface is connectable to a reservoir by way of a travel-responsive controllable valve.
A vehicle hydraulic brake system is known wherein a tandem master cylinder is used for the pressure supply of the wheel brakes. In turn, the tandem master cylinder is actuatable by way of a hydraulic power booster, the pressure chamber of which can be supplied by way of a brake valve with hydraulic pressure which is proportional to the actuating force respectively applied on the brake pedal. The master cylinder piston is designed as a stepped piston and confines with a pedal-close annular surface a chamber which is connectable to an unpressurized supply reservoir by way of a travel-responsive controllable valve. The valve is composed substantially of a bore terminating radially into the master cylinder housing, which bore is normally closed by the larger piston step of the master cylinder piston and which will be released when the master cylinder piston has performed a predetermined axial displacement. The bore leading radially into the master cylinder housing may be considered as breather bore.
By way of valve means controllable by a slip control electronics, there is in the state of brake release and during normal braking operations hydraulic communication between the chamber confined by the annular surface of the master cylinder piston and the working chamber of the master cylinder. This has as a result that the cross-sectional surface of the smaller piston step of the master cylinder piston is exclusively responsible for the pressure generation in the working chamber of the master cylinder. During control action, this will cause isolation of the chamber confined by the annular surface of the master cylinder piston by means of a change-over valve. Simultaneously, the working chamber of the master cylinder is acted upon by the dynamic pressure prevailing in the pressure chamber of the hydraulic power booster. A resetting force is exerted on the master cylinder piston by the pressure now prevailing in the working chamber. In case the bore terminating radially into the master cylinder housing is still closed by the peripheral surface of the master cylinder piston, there will be no resetting of the master cylinder piston. On the other hand, resetting of the master cylinder piston will be effected when the bore terminating radially into the master cylinder housing has already been released by the peripheral surface of the larger piston step of the master cylinder piston. In this event, pressure fluid escapes from the chamber confined by the pedal-side annular surface of the master cylinder piston via the bore terminating radially into the master cylinder housing and via a correspondingly actuated change-over valve into the pressure supply reservoir. The resetting movement will last until the bore terminating radially into the master cylinder housing is closed again by the peripheral surface of the larger piston step of the master cylinder piston. In this operating condition, the chamber confined by the master cylinder piston is again isolated hydraulically.
It is a disadvantage in the described device that the bore terminating radially into the master cylinder housing must have a relatively small cross-section. On the one hand, a relatively small cross-section of the bore terminating into the master cylinder housing has as a consequence that the resetting movement of the master cylinder piston will be damped on pressurization of the working chamber, while on the other hand the wear of the annular seal arranged at the larger piston step of the master cylinder piston will be reduced. In contrast thereto, there is the danger with such throttle bores that they become stuffed inadvertently by dirt particles in the brake system.
It is therefore an object of the present invention to improve upon a hydraulic brake system of the species initially referred to such that malfunctions due to dirt particles in the pressure line system will be excluded to the greatest extent possible.